The present invention relates to a fixing device for fixing a toner image to a sheet material as a target to which the toner image should be fixed, in an image forming apparatus such as an electrostatic copying machine or a laser printer.
A fixing device incorporated in a copying machine using an electrophotographic process fixes a developer which is toner formed on a sheet material by heating and melting the developer. A method using a radiated heat by a halogen lamp (filament lamp) is being widely used as a method for heating toner, which is applicable to the fixing device.
In the method using a halogen lamp as a heat source, the following structure is being widely used. That is, a pair of rollers are provided so that a predetermined pressure can be applied to the sheet material and toner. At least one of the paired rollers is constructed in a hollow columnar shape, and the halogen lamp formed in a columnar shape is provided in the inner space of the roller. In this structure, the roller in which the halogen lamp is provided constructs an acting part (nip) at a position where this roller contacts the other roller, thereby to apply a pressure and a heat to the sheet material and toner which are guided to the nip. That is, the sheet material which is a paper is let pass through a fixing point as a press contact portion (nip) between a heat roller provided with the lamp and a press roller which rotates as a slave to the heat roller. The toner on the paper is thereby melted and fixed to the paper.
In a fixing device using a halogen lamp, light and heat from the halogen lamp are radiated in all circumferential directions so that the roller is heated entirely. In this case, the heat conversion efficiency is 60 to 70% in consideration of the loss caused when converting light into heat and the efficiency at which heat is transferred to the rollers by warming the air in the roller. Thus, it is known that the heat efficiency is low, the power consumption is high, and the warm-up time is long.
Therefore, a fixing device using a cylindrical heatproof film material has been put into practical use, in place of the heat roller and press roller. This structure is constructed by a heat generation member and a heatproof film which moves in tight contact with the heat generation member. Heat energy of the heat generation member is supplied from the film to a sheet material, by moving the heatproof film together with the sheet material with the film kept in tight contact with the heat generation member.
In this fixing device, it is necessary manage the temperature of a linear heat generation member, so that uniformity in manufacture and highly accurate temperature control during operation are required. In addition, the quantity of heat of the heat generation must be set to a high heat quantity in case of a high-speed copying machine. Therefore, the power consumption is so high that the costs cannot be reduced.
A fixing device which uses induction heating has a been proposed as a substitute for the methods using a halogen lamp or a heat-proof film. For example, Japanese Patent Application KOKAI Publication No. 8-76620 discloses an apparatus in which an electrically conductive film is heated by a magnetic field generation means and toner is fixed to a paper kept in tight contact with the conductive film. A heat generation belt (electrically conductive film) is inserted between a member forming part of the magnetic field generation means and a heat roller, thereby forming a nip.
Japanese Patent Application KOKAI Publication No. 9-258586 discloses a method in which a heat generation member having a coil wound around a core provided along the rotation axis of a fixing roller is used and an eddy current is let flow through the fixing roller, thereby to achieve heating.
In case of the fixing device of the induction heating type, a heating coil is used as a magnetic field generation mechanism. Although a method for controlling the temperature of the roller surface has been proposed, only insufficient temperature detection is carried out with respect to the heating coil inside the roller. That is, it is not possible to respond to a case where a part of the roller or film is abnormally heated due to abnormal heat generation of the coil as a heat generation member. Also, it is not respond to another case where a part of the coil is heated by radiation heat from the roller surface. For example, Japanese Patent Application KOKAI Publication No. 9-19785 discloses a structure in which a coil temperature detection means and a fuse are included in a holder which supports a coil. This structure functions without problems if the current flowing through the coil is uniform and the increase of the coil temperature is constant at any places. However, it is not possible to respond to a case where a part of the roller or film is abnormally heated.
This suggests that the temperature of the heat generation member must be managed to be uniform like the above-explained heating method using a film, so it cannot be a fixing device which is advantageous in view of the uniformity in manufacture and the highly accurate temperature control during operation.
That is, in the fixing devices of the induction heating type that have been proposed up to now, a temperature difference appears between a part (paper-passing part) where a paper passes and a part (non-paper-passing part) where no paper passes. The roller surface temperature increases particularly at the non-paper passing part, thereby the temperature increases at coil end portions due to radiation heat from the roller surface. As a result, the coil may receive a heat of a heat-proof temperature or more and may be damaged. Depending on the shape of the coil, the entire circumference of the roller cannot be uniformly heated in the circumferential direction of the roller, and a temperature difference may be caused in the circumferential direction of the roller. This factor restricts heat generation at the above-mentioned coil end portions. Therefore, there have been demands for a coil temperature detection means capable of detecting the temperature.
The present invention has an object of providing a fixing device of an induction heating method, which has a temperature detection means capable of detecting the temperature of a coil regardless of the shape of the coil and which can uniformly heat the entire area of the outer surface of a roller to a uniform temperature within a short time.
The present invention provides a fixing device comprising: an endless member having a metal layer made of a conductive material; an electromagnetic induction coil provided near the endless member, for causing the endless member to generate heat by an alternating current applied to flow through the electromagnetic induction coil; a current control section for controlling the current flowing through the electromagnetic induction coil; a rotation mechanism for rotating the endless member; and a rotation mechanism control section for selectively operating the rotation mechanism.
Also, the present invention provides a fixing device comprising: a first endless member which has a cylindrical or belt-like shape and includes a conductive part; a second endless member which has a cylindrical or belt-like shape, includes a conductive part, and contacts an arbitrary point in a circumferential direction of the first endless member; a coil member provided inside at least one of the first and second endless members, for generating an eddy current at the conductive part of the at least one of the endless members, the coil member making no contact with an inner surface of the at least one of the endless member; a power source circuit connected with an external power source and capable of supplying a current having a predetermined frequency to the coil member; a current control section for controlling a size of the current supplied to the coil member from the power source circuit, and electric-conduction/shut-off of the coil member; a rotation mechanism for rotating the endless members; and a rotation mechanism control section for selectively controlling the rotation mechanism.
Further, the present invention provides an image forming apparatus comprising: a photosensitive member for holding a latent image corresponding to an image to be outputted; a developing device for selectively supplying a visualizing agent to the latent image held by the photosensitive member, thereby to form a visualizing-agent image corresponding to the latent image, on the photosensitive member; a transfer device for transferring the visualizing-agent image formed by the developing device to a transfer medium from the photosensitive member; and a fixing device including a first endless member which has a cylindrical or belt-like shape and includes a conductive part, a second endless member which has a cylindrical or belt-like shape, includes a conductive part, and contacts an arbitrary point in a circumferential direction of the first endless member, a coil member provided inside at least one of the first and second endless members, for generating an eddy current at the conductive part of the at least one of the endless members, the coil member making no contact with an inner surface of the at least one of the endless member, at least two temperature detection devices provided at a predetermined interval in a rotating direction of a metal layer of the at least one of the endless members, for detecting a temperature of the electromagnetic induction coil or a temperature of the metal layer, a power source circuit connected with an external power source and capable of supplying a current having a predetermined frequency to the coil member, a current control section for controlling a size (frequency) of the current supplied to the coil member from the power source circuit, and electric-conduction/shut-off of the coil member, a rotation mechanism for rotating the endless members, and a rotation mechanism control section for selectively controlling the rotation mechanism, wherein the visualizing-agent image transferred to the transfer medium by the transfer device and the transfer medium are heated and pressed between the first and second endless members.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.